nodejs variable in regex match()

Question

how can I use a variable in match() if I want to use regex? This works fine for me:

if (message.match('!' + cmd)) {
  this.client.say(to, myJson[cmd]);
}

but if I try something like this it will not check the variable.

    if (message.match(/!cmd/i)) {
      this.client.say(to, myJson[cmd]);
    }

I already searched for a solution but could only find something for replace.

Edit: it's not a duplicate of How do you use a variable in a regular expression? as it explains it for replace not match.